1. Field of the Invention
The invention relates generally to digital signal processing, and more particularly to a method and apparatus for even harmonic distortion compensation for digital data detection in a direct access storage device (DASD).
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. A slider supports one or more magnetic heads. The slider is lightly biased to cause the heads to move toward the recording surface when the disk is stationary; but as the disk is brought up to operating speed, an air bearing is generated which moves each slider and hence the heads away from the recording surface toward a preselected flying height. Achievement of a higher data density on magnetic disks has imposed increasingly narrow transducing gaps.
A magneto-resistive (MR) transducing head exhibits a change in resistance when in the presence of a changing magnetic field. This resistance change is transformed into a voltage signal by passing a constant current through the MR element. The value of DC voltage, for a given head, is the product of the constant bias current and the total resistance between the head lead terminals. The temperature coefficient of resistivity of the MR material is 0.02%/.degree.C.
Even harmonic distortion in the readback signal for recording heads, particularly heads having magneto-resistive readback elements, causes excessive data error rates if not compensated. A thermal asperity (TA), for example, is a physical defect on a magnetic disk surface that when struck by the magneto-resistive (MR) element on a recording head causes a thermal response. For example, thermal asperities can locally increase the strip temperature by more than 100.degree. C. Many of the TA's are associated with build contamination, and the TA's can grow over time. For example, it has been shown that a small disk surface defect can act as a nucleation site where debris can accumulate over time. This becomes a grown defect and eventually a TA. If not compensated for properly, it will cause customer hard data errors. The readback signal during the TA event shows large even harmonic components associated with the signal baseline shift. As heads fly lower approaching contact recording, TA's will be increasing in frequency and possibly severity. Other head non-linearities that produce baseline shift or signal asymmetry also produce even harmonic distortions that must be compensated to prevent errors which can affect file performance and cause hard data errors. When contact recording is used, MR TA events may be numerous; and the correction techniques applied will be crucial to customer acceptance.
Disadvantages of the known analog arrangements for minimizing the effect of thermal asperities on the read data include the hardware required and the corresponding electronics cost and the required error burst length for a given thermal transient amplitude. Higher data rates will require ever decreasing reaction times necessary for thermal asperity compensation. Known analog techniques will be too slow.